STERLING HEIGHTS, Mich. — A city police officer died Saturday, hours after he was struck by a shotgun blast as he sat in his car, authorities said.

Police had identified a suspect but didn’t know why Officer Mark Sawyers was shot, police Chief Barnett Jones said at a news conference in this Detroit suburb. No one had been arrested by midday Saturday.

“It was a very bold, calculated and cold murder,” said Capt. David Vinson.

Sawyers, 30, had just finished investigating a traffic accident Friday evening and had parked at a shopping center to complete his paperwork when a man fired a shotgun at him from another vehicle, Lt. Michael Reese said.

A shopper called police and said she saw a man lean into the police car’s broken passenger-side window, and then walk to another vehicle. It was still daylight at the time.

Investigators believe the attacker took Sawyers’ service weapon.

Police said they didn’t think the gunman had any connection to Sawyers before the shooting. They weren’t sure whether a second person was involved.

Jones called on the Michigan Legislature to authorize capital punishment for people convicted of murdering on-duty police officers. Efforts to place the issue before voters through the state House have failed twice in the past five years.

Sawyers joined the Sterling Heights police department about seven months ago after three years as an officer in Detroit. Jones said he was the first Sterling Heights killed in a shooting in the line of duty.